The OXBUDS project was funded by the Natural Environment Research Council (NERC) with the grant reference - NE/M003248/1 - and was lead by Professor Claire Reeves (University of East Anglia). This project brought together experts in firn air data interpretation with experts in chemistry-climate modelling. Both groups also had considerable expertise in organic (including alkyl) nitrate chemistry. OXBUDS specifically builds on past NERC funded work on the trends of alkanes and alkyl nitrates in firm air using simply relationships and models.

The hydroxyl radical (OH) is the dominant oxidizing agent in the troposphere, as such its concentration controls the abundances and lifetimes of most atmospheric pollutants, including the important greenhouse gas methane (CH4). Ozone (O3) is also an important oxidant and is itself a greenhouse gas. The concentrations of OH and O3 are interdependent, both being determined by a complex series of reactions involving CH4, carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOX = NO + NO2). As emissions of these compounds have changed substantially since pre-industrial times, the tropospheric budgets of OH and O3 will also have changed. However, there are large uncertainties associated with current understanding of these past changes and consequently very large uncertainties in projected future changes and associated climate impacts.

Most of this uncertainty in past trends comes from lack of observations to constrain studies. Whilst there are a few direct observational data sets which indicate how O3 concentrations changed through the 20th century, there are none for OH. Direct observational data sets of CH4, NMVOCs, CO and NOX, extend, at best, from the 1980s. These time series can be extended backward in time through the analysis of air trapped in firn (unconsolidated snow). Whilst such historic time series have been available for CH4 for some time, only recently have they become available for CO and for some NMVOCs, in particular alkanes. Furthermore, we have also recently determined, from firn analysis, historic time series of alkyl nitrates. Alkyl nitrates are products of the chemistry involving NOX and as such can be used as a diagnostic of the changes in NOX.

These new (and in the case of the alkyl nitrates, unique), historic time series provide an exciting opportunity to investigate the changing OH and O3 budgets of the northern hemisphere troposphere since 1950 with observational constraints never available before. Very interestingly, the simple analyses carried out on these time series to date suggest that substantial changes in the atmospheric chemistry have occurred. To exploit the full value of these time series a detailed study is required with a comprehensive chemistry-climate model which is where the OXBUDS project comes in.

The aim of this project was to use long term trends of alkane and alkyl nitrate concentrations to determine the impact of changing anthropogenic emissions on the ozone and hydroxyl radical (OH) budgets of the northern hemisphere troposphere since 1950.

The outcomes of this study were: 1) a better understanding of the impact of changing anthropogenic emissions on the OH and ozone budgets of the northern hemisphere troposphere; 2) an improved modelling capability with which to project future changes and better inform climate policy.

This project had several specific objectives which were to:

1) Significantly improve the alky nitrate chemistry in the chemistry-climate model UM-UKCA (Unified Model - UK Chemistry Aerosol).
2) Determine the impact of different emission regions on Arctic alkane and alkyl nitrate composition and how this may have changed since 1950.
3) Thoroughly evaluate the UM-UKCA model against a variety of present-day alkane and alkyl nitrate data sets and, in doing so, improve understanding of factors controlling the current oxidant chemistry of the northern hemisphere troposphere.
4) Employ UM-UKCA to investigate the historical trends in tropospheric composition and chemistry required to reproduce the firn-derived time series in alkanes and alkyl nitrates, and, in doing so, determine the changes in OH and ozone budgets of the northern hemisphere troposphere.
5) Perform attribution studies to assess the influence of emissions of individual compounds on the modelled, historical changes in OH and ozone.
6) Compare the results from this study with data previously generated from a number of other chemistry-climate models.
7) Make recommendations regarding further development of chemistry-climate models, in particular improvements in modelling ozone via the inclusion of alkyl nitrate chemistry.
8) Maximise the feed through of the results into future IPCC assessments and ultimately international policy.

This project ran a set of modelling experiments to test the impact of primary (marine and biomass burning) alkyl nitrate emissions and secondary formation of alkly nitrates from peroxy radicals on the ozone, NOx and NOx reservoirs. It used UM-UKCA vn7.3 StratTrop (CheST) chemistry similar to that briefly documented In Banerjee et al. (DOI:10.5194/acp-14-9871-2014), marine emissions from Fisher et al (DOI: 10.5194/acp-16-5969-2016) and biomass burning emissions derived from GFED vn4. Other inputs are as for AR5 in Banerjee. The experiments describe the impact of methyl, ethyl and n- and i-propyl nitrates. The data from these experiments are voluminous and specific to these model runs and tehrefore remain with the project team. They are in native UM PP file format, and are output on the native N48L63 grid, 2.5x3.75 degree spatial resolution, with 63 model levels up to 85km, and comparable after interpolation to e.g. recent ATOM flights. Data are not stored at CEDA but can be accessed via the project contact: paul.griffiths@ncas.ac.uk